where is the discriminant, which is a resolvent for the alternating group. In the case of a cubic equation, this resolvent is sometimes called the quadratic resolvent; its roots appear explicitly in the formulas for the roots of a cubic equation.
The Cayley resolvent is a resolvent for the maximal resoluble Galois group in degree five. It is a polynomial of degree 6.
These three resolvents have the property of being always separable, which means that, if they have a multiple root, then the polynomial p is not irreducible. It is not known if there is an always separable resolvent for every group of permutations. For every equation the roots may be expressed in terms of radicals and of a root of a resolvent for a resoluble group, because, the Galois group of the equation over the field generated by this root is resoluble.
Definition
Let be a positive integer, which will be the degree of the equation that we will consider, and an ordered list of indeterminates. This defines the generic polynomial of degree where is the ithelementary symmetric polynomial. The symmetric group acts on the by permuting them, and this induces an action on the polynomials in the. The stabilizer of a given polynomial under this action is generally trivial, but some polynomials have a bigger stabilizer. For example, the stabilizer of an elementary symmetric polynomial is the whole group. If the stabilizer is non-trivial, the polynomial is fixed by some non-trivial subgroup ; it is said an invariant of. Conversely, given a subgroup of, an invariant of is a resolvent invariant for if it is not an invariant of any bigger subgroup of. Finding invariants for a given subgroup of is relatively easy; one can sum the orbit of a monomial under the action of. However it may occur that the resulting polynomial is an invariant for a larger group. For example, consider the case of the subgroup of of order 4, consisting of,, and the identity. The monomial gives the invariant. It is not a resolvent invariant for, as being invariant by, in fact, it is a resolvent invariant for the dihedral subgroup, and is used to define the resolvent cubic of the quartic equation. If is a resolvent invariant for a group of index, then its orbit under has order. Let,..., be the elements of this orbit. Then the polynomial is invariant under. Thus, when expanded, its coefficients are polynomials in the that are invariant under the action of the symmetry group and thus may be expressed as polynomials in the elementary symmetric polynomials. In other words, is an irreducible polynomial in whose coefficients are polynomial in the coefficients of. Having the resolvent invariant as a root, it is called a resolvent. Consider now an irreducible polynomial with coefficients in a given field . If the Galois group of is contained in, the specialization of the resolvent invariant is invariant by and is thus a root of that belongs to . Conversely, if has a rational root, which is not a multiple root, the Galois group of is contained in.
Terminology
There are some variants in the terminology.
Depending on the authors or on the context, resolvent may refer toresolvent invariant instead of to resolvent equation.
A Galois resolvent is a resolvent such that the resolvent invariant is linear in the roots.
A relative resolvent is defined similarly as a resolvent, but considering only the action of the elements of a given subgroup of, having the property that, if a relative resolvent for a subgroup of has a rational simple root and the Galois group of is contained in, then the Galois group of is contained in. In this context, a usual resolvent is called an absolute resolvent.
Resolvent method
The Galois group of a polynomial of degree is or a proper subgroup of that. If a polynomial is separable and irreducible, then the corresponding Galois group is a transitive subgroup. Transitive subgroups of form a directed graph: one group can be a subgroup of several groups. One resolvent can tell if the Galois group of a polynomial is a subgroup of given group. The resolvent method is just a systematic way to check groups one by one until only one group is possible. This does not mean that every group must be checked: every resolvent can cancel out many possible groups. For example, for degree five polynomials there is never need for a resolvent of : resolvents for and give desired information. One way is to begin from maximal subgroups until the right one is found and then continue with maximal subgroups of that.
